1. Field of the Invention
This invention relates to a misfire state discrimination system of an internal combustion engine, particularly to a misfire state discrimination system of a vehicle internal combustion engine.
2. Description of the Prior Art
In discriminating the misfire state of an internal combustion engine, the number of misfires detected during a prescribed number of consecutive ignitions (combustions) is counted and compared with a reference value to discriminate, for example, whether the engine is in a misfire state (has a misfire rate) that degrades the catalyst of the catalytic converter or is in a misfire state that markedly fouls the exhaust gas.
Technologies of this type are taught by, for example, Japanese Laid-open Patent Application Nos. Hei 7(1995)-63110, 7(1995)-259632, and 4(1992)-209949. These conventional methods judge a misfire state (misfire rate) by discriminating or determining misfiring from fluctuations in engine speed. Another method taught by Japanese Laid-open Patent Application No. Hei 5(1993)-164033 discriminates or determines the misfiring by judging based on the secondary voltage of the ignition system whether or not ion current is flowing across the spark plug gaps.
Since catalyst degradation accelerates sharply when the ambient temperature approaches the melting point of the catalyst (and the metal contained in the catalyst carrier), a requirement of a misfire state discrimination system is that it be capable of quickly discriminating a misfire state (misfire rate) resulting in such a temperature so as to prevent rapid catalyst degradation. Moreover, since a misfire rate of only a few percent sharply degrades the composition of the exhaust gas, even minor misfire states (misfire rates) must be discriminated with high accuracy so that corrective measures can be implemented. Rapid and high-accuracy discrimination of internal combustion engine misfire state (misfire rate) requires the number of misfires among an adequate number of firings discriminated for misfire to be determined in a short time period.
On the other hand, misfire discrimination or determination is impossible during many operating states or conditions of an internal combustion engine. For instance, misfire discrimination is impossible when the fuel supply is cut off, when the engine is being driven from the wheel side (during engine braking), during transient operation such as when the engine speed is rising or falling rapidly, and when engine output is being forcibly changed by external factors such as the wheels.
As seen, for example, in Japanese Laid-open Patent Application Nos. Hei 7-63110, 7-259632 etc. cited above, the prior art responds to the transition of engine operation to one of these states or conditions not allowing misfire discrimination by clearing the parameters calculated up to that time, including the number of ignitions discriminated for misfire, (i.e., by initializing the system).
When operating states or conditions that prevent misfire discrimination occur frequently, such as when the engine speed is repeatedly increased and decreased, therefore, the prior art needs a long time for misfire discrimination and cannot effect the discrimination rapidly and accurately.
An object of this invention is therefore to overcome the aforesaid drawbacks by providing a misfire state discrimination system of an internal combustion engine that, even in cases of frequent occurrence of operating states in which misfire cannot be discriminated or determined, enables a rapid and accurate discrimination of a misfire state upon return to an operating state or condition in which discrimination or determination of misfire is possible.
The conventional technologies discussed above also require a long period of time for misfire discrimination or determination and are incapable of rapid and accurate misfire state discrimination in cases where the engine is started and stopped repeatedly within a short time period.
Another object of the invention is to provide a misfire state discrimination system of an internal combustion engine that enables rapid and accurate discrimination of misfire state even when the engine is started and stopped repeatedly within a short time period.
This invention achieves these objects by providing, in one aspect, a system for discriminating a misfire state of an internal combustion engine, including engine operating condition determining means for determining whether an operation of the engine is under a condition that makes misfire determination possible, misfire determining means for determining whether a misfire has occurred in the engine when the engine operation is under the condition that makes misfire determination possible, combustion counting means for counting a number of combustions of the engine during which said misfire determining means determines whether the misfire has occurred, and misfire counting means for counting a number of misfires determined by said misfire determining means, and misfire state discriminating means for comparing the number of misfires counted by said misfire counting means with a reference value and for discriminating a misfire state of the engine based on a result of the comparison. In the system number saving means is provided for saving at least one of the number of combustions counted by said combustion counting means and the number of misfires counted by said misfire counting means, when the engine operation moves from the condition that makes misfire detection possible.